1. Field of the Invention
The present invention relates to compositions of dielectric fluids, useful particularly as insulating fluids and coolants in electrical devices such as, for example, transformers, capacitors, cables, or circuit breakers
2. Description of the Related Art
Dielectric fluids compositions are commonly used in electrical devices, particularly in transformers. These liquids have the aim to isolate the various conductive elements of the device and to limit the heating of the device during its operation, and increase the useful life duration of the device. A performant fluid is essential to minimize the size of the device.
Much of the currently known dielectric liquid compositions are compositions comprising essentially hydrocarbons from specific cuts of crude oils. These compositions based on petroleum derivatives generally have a low ignition temperature (and therefore a significant flammability) and are also most often characterized by low biodegradability, if not also characterized by a relatively high toxicity (dielectric compositions based on petroleum derivatives have indeed, in most cases, aromatic molecules and/or halogen). These liquids are found to be relatively unsatisfactory for insulation of electrical devices. Indeed, especially when used in electrical transformers where large amounts of heat can be generated during operation, these fluids present significant flammable risks. In addition, in case of breakage of the envelope of the electrical device, liquid-based petroleum derivatives are likely to cause significant pollution (soil, rivers, ground water). In case of fire due to inflammation of these compounds, air pollution may also occur.
In addition to these dielectric fluids containing petroleum derivatives, one has developed other less toxic liquid compositions such as silicone oils, for example. These alternative compositions certainly prove more effective, but they have a much higher cost, and they are used in practice only in some specific appliances, with high-value, such as, for example traction transformers or electrical equipment installed in buildings of significant height and/or open to the public.
Therefore, to replace the dielectric fluids containing petroleum derivatives, one has sought to use compounds with interesting dielectric performances, which are nontoxic and biodegradable, and the synthesis of which is the cheapest possible. In this context, one has proposed including natural plant oils or animal waste fats as potential candidates, since some of them present some interesting insulation properties.
However, it has been found that natural plant oils cannot be used, in a large number of cases, as such as dielectric fluids in electrical devices such as, for example, transformers. Indeed, they have certain disadvantages, including high viscosity (especially at low temperatures) associated with a high pour point, and in general, they also may exhibit an important tendency to oxidation (oxidability) pending on their source, refining or handling in general.
To improve the characteristics of vegetable oils it has been proposed (in international applications WO 97/22977, WO 97/49100, WO 98/31021 and WO 00/11682) to modify these oils, by adding antioxidants, antifreeze, or viscosity improvers such as polymethacrylates, to counteract the aforementioned defects. However, these additives are also generally low or non-biodegradable or toxic, they prove to be damaging to electrical devices in which they are used. This is particularly the case of polymethacrylates, known for its acidity, which can therefore lead to significant corrosion in electrical devices, and particularly in electrical transformers.
To a lesser extent, attempts have been made to change the triglycerides (esters of glycerol and fatty acids) present in natural vegetable oils, in order to modify the properties of such vegetable oils without losing their biodegradability. In this regard, various attempts have been made to convert these esters of glycerol in esters of other alcohols, such as esters of isobutanol. These tests are however not fully conclusive: indeed, the works done in this particular context have failed to obtain compositions having all the physical and chemical properties that such a composition should have for an efficient and sustainable use in an electrical device. In particular, the tests performed so far to change the triglycerides of natural vegetable oils have failed to produce compositions with all the required characteristics for use as cooling dielectric fluid in a dielectric device. In particular, one has never described compositions of modified vegetable oils from a transformation of present triglycerides, which have properties adapted both in terms of electrical insulation, cooling properties, low viscosity, low density and stability vis-à-vis oxidation, as required by international standards IEC 60296, IEC 60465, or IEC 61099 and equivalent.
WO 2004/108871 discloses liquid compositions that are based on modified oleic canola oil and are used as insulating liquids and heat transfer liquids, and electrical devices containing said liquid compositions. In this prior publication, the liquid composition is based on a mixture of natural triglycerides and fatty acid esters of 2-ethyl-1-hexanol, said fatty acids usually derived from a vegetable oil, the mixture may optionally contain other esters, in which:    (i) the fatty acids present in all the esters of glycerol (triglycerides) and esters of 2-ethyl-1-hexanol mixture include, by weight with respect to the total mass of fatty acids in the esters:            at least 70%, and preferably between 72 and 90% monounsaturated fatty acids in C18, such as, for example, oleic acid        between 10 and 20% of polyunsaturated fatty acids, the polyunsaturated fatty acids are generally mostly polyunsaturated fatty acids in C18, such as linoleic acid and/or linolenic acid        less than 10% and preferably less than 7% of fatty acids having a hydrocarbon chain containing a number of different carbon atoms of 18, and            (ii) the triglycerides present in the mixture corresponds to a mass of 25 to 80%, preferably between 40 and 75% compared to the total mass of the mixture, the fatty acid esters other than triglycerides present in said composition being, in general, essentially mono-esters of 2-ethyl-1-hexanol.
The main idea in this prior publication, which is further discussed below, was that by modifying some specific natural vegetable oils by converting the triglycerides of fatty acids present in a fraction of these oils in particular esters of an alcohol, namely 2-ethyl-1-hexanol, one obtained electrically insulating liquid compositions which, subject to control the final content of esters of 2-ethyl-1-hexanol, proved well suited as a dielectric fluid coolant for electrical devices, especially according to the criteria of these standards. Unexpectedly, these modified oils obtained after modification of a fraction of the oil with 2-ethyl-1-hexanol have interesting properties, especially in terms of electrical insulation and viscosity. Thus, the compositions obtained had proven to possess high breakdown voltages and low viscosities.
WO 2010/111698 discloses the use of soy oil as a heat-transfer dielectric fluid in a device to generate, store, convert and/or distribute electrical energy wherein the soy oil is one in which at least 70% of the fatty acids are C14 to C22 mono-unsaturated and less than 16% of the fatty acids are polyunsaturated.
A first example of a method for the extraction and the processing of soybean seeds to produce soybean oil and meal has been described in WO 2010/111698.
Another example of a method for producing fatty acid esters from flattened oleaginous grains has been disclosed in WO 2009/013349.
More specifically, the process taught by this prior publication is used to prepare fatty acid esters which can be used as biodiesel, prepared from whole oilseeds, and comprises the following steps:    a. Preheating the whole non-hulled seeds;    b. Flattening the oilseeds with their husks;    c. Drying the flattened seeds so as to obtain a water and volatile matter content of between 0.5 and 2.5%, preferably between 1.5% and 2%;    d. Transesterification by contacting the flattened, dried seeds with an alcohol medium in the presence of a catalyst;    e. Separating the liquid and solid phases resulting from transesterification;    f. Neutralizing the liquid phase derived from step e);    and    g. Removing the alcohol and separating the glycerin from the fatty acid esters.
Document WO 2010/124118, on the other hand discloses a process and device for making grease. In this publication, it has in particular been shown that vegetable oils have a uniquely different behavior when exposed to high temperatures. In the case of some vegetable oils, once the oil temperature exceeds 150° C. (300° F.), the oil begins to oxidize rapidly and if steps are not taken to remedy this rapid oxidation, the product will begin to polymerize, resulting in irreversible change. In such cases, the product could partially or fully polymerize or change state from a soap into a polymer with no or little lubrication value. But, several methods exist for stabilizing soybean or other vegetable oils so they can be reacted with lithium and produce stable greases. The use of high oleic vegetable oils is often employed to improve the oxidation stability of the final product. Vegetable oils, due to their higher viscosity index, present a more stable body when exposed to high temperatures. As a result, properly formulated vegetable oil-based grease would show more stable body in use.
In this prior publication, the Applicant has discovered, inter alia, that a soap, and in turn, grease of the described invention can provide an optimal combination of properties as compared to comparable soaps and greases prepared using conventional heating methods (e.g., hot plate, thermal blankets). The ability to provide more uniform and controllable heating can, in turn, help to make the entire process both faster and more efficient as well. More specifically, it has been described that heating with microwave energy imparts significantly less oxidative damage to a composition as compared to the same composition when heated using conventional means. For instance, the term OSI (for “Oil Stability Index”) is a value that often corresponds with the ability of an oil composition to resist oxidation. A suitable method for determining OSI is known as the AOCS test method Cd 12-92, the disclosure of which is incorporated in this prior publication by reference. In turn, it is typically the fact that the higher the OSI, generally the better suited the composition will be for use, both in terms of initially preparing a soap, and also in terms of using the soap or corresponding grease over a longer period of time. To summarize, it has been shown in this prior publication that the use of microwaves to heat oils, typically oils with high oleic acid content, allows them to maintain their OSI high, in other words their ability to resist oxidation relative to their use as greases especially. The suitability of the OSI testing and relevance to the real performance in use of transformers remain unestablished.
For the electrical purpose of the present invention, fluids based on the use of vegetable oils traditionally use refined, bleached, deodorized and winterized (RBD) oils; therefore all prior usage is established on that basis. In traditional RBD operations. impurities are removed at various steps namely degumming, neutralizing, washing, drying, bleaching, filtering and deodorizing. This chemical process has many drawbacks, such as high energy demand, disposal of polluted effluents but also loss of essential components of the oils which have protective attributes in terms of the durability and preservation of the oil in food or industrial applications. In addition, RBD refineries are very large plants, which may not be suited to industrial applications of the invention.